The present invention relates to solar energy modules and more particularly to solar insulating blocks for use in solar greenhouses and other structures.
Many buildings such as greenhouses require roof and/or window coverings that will admit sunlight. It is generally desirable for these transparent or translucent light-admitting sections to have low thermal conductivity in order to provide a savings in heating or cooling costs within the structure. There are a number of low density or cellular materials with high insulative values (commonly measured in terms of "r" value, and defined as the reciprocal of the thermal conductance measured in BTU/hr/ft.sup.2 /per given thickness of insulating material) but which do not transmit light. On the other hand, a single layer of glass or clear plastic transmits light, but offers little insulation value. Where greater insulation value is required, yet it is necessary to retain the light-transmissive properties of the roof or window, it is common practice to use a double or triple layer of glass or plastic having a small air space between the layers. This technique adds significantly to reducing heat transfer, but numerous applications demand values approaching zero heat transfer.
A further move toward high insulation lead to the development of movable insulation. Movable insulation employs techniques which recognize that the sun only shines during the day, and that additional, non-transparent insulation may be used at night. Such techniques combined maximal nightime insulation with ease of insulation mobility, ranging from simple shutters, shades, drapes or special curtains which may be closed either manually or automatically, to more exotic techniques having double glazing and a relatively deep inner chamber into which can be blown insulation such as polystyrene beads, to more complex and expensive solar panels.
Using added conventional insulation at night has an economical impact on heating costs for the structure's interior, but still suffers from the disadvantage of not providing sufficient daytime insulation. Despite the substantial body of prior art in the field of solar panels, such devices have not gained widespread acceptance because they are expensive, complex and thermally inefficient.
In theory, heat transfer, or its reciprocal, thermal insulation, is simple. Heat energy moves from a region of higher temperature to one of lower temperature by only two means, conduction and radiation. Conduction is the energy transfer that takes place between two masses that are in direct contact with one another, and takes several forms. One form, convection, is so common that it is often definitionally included as a third form of heat transfer. Convection is the more or less circular air flow and consequently hastened heat conduction transfer facilitated by fluid between two regions of dissimilar temperature. The convection phenomenon explains why the air space between double glazing offers diminishing insulative values as the space is widened beyond one or two centimeters. Radiation is the energy given off in electromagnetic form that all bodies above absolute zero emit. Thus, radiation requires no medium of transfer.
While various materials have various heat conductivity and consequently differing insulative values, all materials conduct at least some heat. Heat conduction is zero only in a vacuum. A vacuum, then, represents the theoretical maximum conductive insulation. Radiation, however, flows freely, even across a vacuum. It can, however, be blocked by a shiny, reflective surface. A totally reflective surface therefore represents the theoretical maximum radiation insulation, and a barrier which is both evacuated and reflective represents maximum total insulation. This concept was first put to practical use by Sir James Dewar in 1890 in the vacuum flask or thermos bottle which is still in common usage today both on a small scale for maintaining the temperature of foods and beverages and on a large scale in the cryogenics industry.
There have been several attempts to put this concept to use in the field of solar energy and solar panels. While prior art exists on solar panels which utilize a vacuum, (see for example U.S. Pat. No. 2,918,023) widespread use has not been achieved.
The solution to glazing large, relatively flat areas such as large tracts of farmland, for daytime solar gain and nightime maximum insulation has gone unanswered until the advent of the present invention for two main reasons. First, the tremendous force of full atmospheric pressure against a large, flat surface creates difficult structural problems. For example, two one-meter square plates bounding a 1.times.100.times.100 cm space must withstand roughly ten metric tons of force. Second, any surface which reflects a high percentage of radiant heat will not transmit light.
The art is replete with patents for a wide variety of solar panels and structures such as greenhouses. See for example, U.S. Pat. Nos.: 4,051,832; 4,173,969; 4,178,909; 4,195,441; 4,279,243; 4,267,218; 4,292,955; 4,194,491 4,273,098 and 2,918,023. None, however, have suggested an adequate solution to the above problems.
Structures, such as solar greenhouses are particularly valuable in harsh climates where crops are routinely lost to the vagaries of weather conditions. A particular need for an efficient solar greenhouse which can be economically, simply and efficiently placed over large areas of farmland, is found in harsh climates such as Wisconsin where expensive crops having a long growing season, such as ginseng which has a four year growing cycle from planting to harvest, can be economically disastrous for the farmer. In such harsh winter climates, it is critical to provide for maximum daytime solar gain and maximum nightime insulation to prevent heat loss and temperature drops when sunlight is not available to warm the structure housing the crops. In addition, it is important to protect seedlings from excessive moisture and disease. Is therefore desirable to provide an improved solar energy module which overcomes most if not all of the problems of the prior art.